Internal combustion engines typically use either a mechanical, electrical or hydro-mechanical valve actuation system to actuate the engine valves. These systems may include a combination of camshafts, rocker arms and push rods that are driven by the engine's crankshaft rotation. When a camshaft is used to actuate the engine valves, the timing of the valve actuation may be fixed by the size and location of the lobes on the camshaft.
For each 360 degree rotation of the camshaft, the engine completes a full cycle made up of four strokes, i.e., expansion, exhaust, intake, and compression. Both the intake and exhaust valves may be closed, and remain closed, during most of the expansion stroke, when the piston is traveling away from the cylinder head and the volume between the cylinder head and the piston head is increasing. During positive power operation, fuel is burned during the expansion stroke and positive power is delivered by the engine. The expansion stroke ends at the bottom dead center (BDC) point, at which time the piston reverses direction. The exhaust valve may be opened for a main exhaust event prior to BDC. A lobe on the camshaft may be synchronized to open the exhaust valve for the main exhaust event as the piston travels upward and forces combustion gases out of the cylinder. Near the end of the exhaust stroke, another lobe on the camshaft may open the intake valve for the main intake event at which time the piston travels away from the cylinder head. The intake valve closes and the intake stroke ends when the piston is near bottom dead center. Both the intake and exhaust valves are closed as the piston again travels upward for the compression stroke.
The main intake and main exhaust valve events are required for positive power operation of an internal combustion engine. Additional auxiliary valve events, while not required, may be desirable. For example, it may be desirable to actuate the intake and/or exhaust valves during positive power or other engine operation modes for compression-release engine braking, bleeder engine braking, exhaust gas recirculation (EGR), brake gas recirculation (BGR), or other auxiliary intake and/or exhaust valve events.
With respect to auxiliary valve events, flow control of exhaust gas through an internal combustion engine has been used in order to provide vehicle engine braking. One or more exhaust valves may also be selectively opened to convert, at least temporarily, the engine into an air compressor for engine braking operation. This air compressor effect may be accomplished by either opening one or more exhaust valves near piston top dead center position for compression-release type braking, or by maintaining one or more exhaust valves in a relatively constant cracked open position during much or all of the piston motion, for bleeder type braking. In both types of braking, the engine may develop a retarding force that may be used to help slow a vehicle down. This braking force may provide the operator with increased control over the vehicle, and may also substantially reduce the wear on the service brakes. Compression-release type engine braking has been long known and is disclosed in Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is hereby incorporated by reference.
Generally, engine braking systems may control the flow of exhaust gas to incorporate the principles of compression-release braking, bleeder braking, exhaust gas recirculation, and/or brake gas recirculation. During compression-release engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, a power producing internal combustion engine into a power absorbing air compressor. As a piston travels upward during its compression stroke, the gases that are trapped in the cylinder may be compressed. The compressed gases may oppose the upward motion of the piston. As the piston approaches the top dead center position, at least one exhaust valve may be opened to release the compressed gases in the cylinder to the exhaust manifold, preventing the energy stored in the compressed gases from being returned to the engine on the subsequent expansion down-stroke. In doing so, the engine may develop retarding power to help slow the vehicle down.
During bleeder engine braking, in addition to, and/or in place of, the main exhaust valve event, which occurs during the exhaust stroke of the piston, the exhaust valve(s) may be held slightly open during the remaining three engine cycles (full-cycle bleeder brake) or during a portion of the remaining three engine cycles (partial-cycle bleeder brake). The bleeding of cylinder gases in and out of the cylinder may act to retard the engine. Usually, the initial opening of the braking valve(s) in a bleeder braking operation is in advance of the compression top dead center, i.e., early valve actuation, and then lift is held constant for a period of time. As such, a bleeder type engine brake may require lower force to actuate the valve(s) due to early valve actuation, and generate less noise due to continuous bleeding instead of the rapid blow-down of a compression-release type brake.
Exhaust gas recirculation (EGR) systems may allow a portion of the exhaust gases to flow back into the engine cylinder during positive power operation. EGR may be used to reduce the amount of NO created by the engine during positive power operations. An EGR system can also be used to control the pressure and temperature in the exhaust manifold and engine cylinder during engine braking cycles. Generally, there are two types of EGR systems, internal and external. External EGR systems recirculate exhaust gases back into the engine cylinder through an intake valve(s). Internal EGR systems recirculate exhaust gases back into the engine cylinder through an exhaust valve(s) and/or an intake valve(s).
Brake gas recirculation (BGR) systems may allow a portion of the exhaust gases to flow back into the engine cylinder during engine braking operation. Recirculation of exhaust gases back into the engine cylinder during the intake stroke, for example, may increase the mass of gases in the cylinder that are available for compression-release braking. As a result, BGR may increase the braking effect realized from the braking event.
Many different actuation systems may be used to selectively actuate engine valves to produce brake gas recirculation and compression-release events. One known type of actuation system is a lost motion system, described in the above-referenced Cummins patent. Another example of a lost motion system for variable valve actuation is disclosed in Vanderpoel, et al., U.S. Pat. No. 7,152,576 (Dec. 26, 2006), which is hereby incorporated by reference. An example of a system with primary and offset actuator rocker arms for engine valve actuation is disclosed in Janak, et al., U.S. Pub. No. 2006/0005796 (Jan. 12, 2006), which is hereby incorporated by reference.
In many internal combustion engines, the intake and exhaust valves may be actuated by fixed profile cams, and more specifically, by one or more fixed lobes or bumps that are an integral part of each cam. The cams may include a lobe for each valve event that the cam is responsible for providing. The size and shape of the lobes on the cam may dictate the valve lift and duration which result from the lobe. For example, an exhaust cam profile for a system may include a lobe for a brake gas recirculation event, a lobe for a compression-release event, and a lobe for a main exhaust event.
It may also be desirable to increase the exhaust back pressure in the exhaust manifold during engine braking. Higher exhaust back pressure may increase gas mass and pressure in the engine cylinder available for engine braking, and thereby increase braking power. Increased exhaust back pressure, however, may undesirably increase the force required to open the exhaust valve for a compression-release event because the opening force applied to the exhaust valve must exceed the increased pressure in the engine cylinder resulting from the increased exhaust back pressure. To some extent the increased exhaust back pressure may also increase the pressure applied to the back of the exhaust valve, which may counter-balance the increased pressure in the cylinder and thus reduce the loading on the exhaust valve opening mechanism used for the compression-release event.
Increasing the pressure of gases in the exhaust manifold may be accomplished by restricting the flow of gases through the exhaust manifold. Exhaust manifold restriction may be accomplished through the use of any structure that may, upon actuation, restrict all or partially all of the flow of exhaust gases through the exhaust manifold. The exhaust restrictor may be in the form of an exhaust engine brake, a turbocharger, a variable geometry turbocharger, a variable geometry turbocharger with a variable nozzle turbine, and/or any other device which may limit the flow of exhaust gases.
Exhaust brakes generally provide restriction by closing off all or part of the exhaust manifold or pipe, thereby preventing the exhaust gases from escaping. This restriction of the exhaust gases may provide a braking effect on the engine by providing a back pressure when each cylinder is on the exhaust stroke. For example, Meneely, U.S. Pat. No. 4,848,289 (Jul. 18, 1989); Schaefer, U.S. Pat. No. 6,109,027 (Aug. 29, 2000); Israel, U.S. Pat. No. 6,170,474 (Jan. 9, 2001); Kinerson et al., U.S. Pat. No. 6,179,096 (Jan. 30, 2001); and Anderson et al., U.S. Pat. Appl. Pub. No. US 2003/0019470 (Jan. 30, 2003) disclose exhaust brakes for use in retarding engines.
Turbochargers may similarly restrict exhaust gas flow from the exhaust manifold. Turbochargers often use the flow of high pressure exhaust gases from the exhaust manifold to power a turbine. A variable geometry turbocharger (VGT) may alter the amount of the high pressure exhaust gases that it captures in order to drive a turbine. For example, Arnold et al., U.S. Pat. No. 6,269,642 (Aug. 7, 2001) discloses a variable geometry turbocharger where the amount of exhaust gas restricted is varied by modifying the angle and the length of the vanes in a turbine. An example of the use of a variable geometry turbocharger in connection with engine braking is disclosed in Faletti et al., U.S. Pat. No. 5,813,231 (Sep. 29, 1998), Faletti et al., U.S. Pat. No. 6,148,793 (Nov. 21, 2000), and Ruggiero et al., U.S. Pat. No. 6,866,017 (Mar. 15, 2005), which are hereby incorporated by reference.
Over the years there have been improvements to lost motion systems for engine braking and there continues to be a need for improvements as technology evolves and new problems are discovered. Improvements are needed for many reasons, including providing a mechanically-driven exhaust main event for cold start and failsafe modes, meeting loading limits, (e.g., cam Hertz stress), avoiding separation and impact loading between cams and rollers, avoiding bridge tilt, meeting exhaust valve seating velocity limits, and protecting against valve-piston contact. There is a risk of valve-piston contact in many electronically-controlled variable valve actuation (VVA) systems. For example, lost motion VVA systems that provide early valve opening and spill oil near peak lifts have an increased risk of valve piston contact if the spill does not function, which may occur, for example, due to a clogged spill port or a broken valve spring. The valve/cam lift ratio of a rocker-actuated VVA system is more limited by the valve-train layout than that of a master-slave system, where the valve/cam lift ratio is governed by hydraulic piston diameters.